1. Field
The present invention relates to cable television distribution networks, and more specifically to canceling intermodulation products within communications signals transmitted on cable television distribution networks.
2. Background
A downstream cable TV plant typically carries tens to hundreds of carrier signals used to transmit television, Internet and other data information. These carriers are arranged in a frequency division multiplexed (“FDM”) format with six megahertz (“Mhz”) spacing between carriers in North America and eight Mhz spacing in Europe.
A cable plant contains non-linear elements, such as amplifiers and laser drivers that introduce intermodulation (“IM”) interference. When the carrier signals pass through such non-linearities, intermodulation interference is generated within the carrier signals. This interference is referred to as IM products. The IM products occur in frequencies which are the sum and difference of multiples of carrier frequencies and generally fall in the bands of the desired signals, where they interfere with the desired signals.
IM products are typically time varying within a signal envelope. The IM spectrum may contain a series of narrow lines, especially if the plant contains many analog TV carriers such as in the case of a National Televison System Committee (“NTSC”) formatted signal. The NTSC standard for television defines a composite video signal with a refresh rate of 60 half-frames (interlaced) per second. Each frame contains 525 lines and can contain 16 million different colors. The IM spectrum may be spread (i.e., not predominated by spectral lines) if the plant contains mostly digital carriers. Some of the digital carriers may be data-bearing signals, such as Data Over Cable Service Interface Specification (“DOCSIS”) signals. DOCSIS signals are used to support cable modem data transmission. Second order IM products are referred to as composite second order (“CSO”) products and third order IM products are referred to as composite triple beat (“CTB”) products.
Because the IM products interfere with a desired signal, methods to remove or attenuate the IM interference are needed. Typical approaches attempt to address this problem at the transmitter end of a cable plant. These approaches often assume the availability of original undistorted data or knowledge of the nonlinearity. These systems are limited in their ability to remove or attenuate IM interference because often a readily accessible version of the original undistorted signal and/or knowledge of the nonlinearity transfer characteristics are not well known.
What are needed are methods to reduce IM interference that do not require knowledge of the non-linearity and access to an original undistorted signal.